Krista Lister testified that Nancy came to her wedding in 2001 with another man.
"She had disclosed that she had met somebody," said Lister.
Lister said she told her sister she disapproved.
Lister said the incident happened in the first year of the Cooper marriage and Nancy was seriously considering not returning to Cary to be with her husband. Lister described Nancy as unhappy with her marriage and that she had no car and felt cut off. She said Nancy returned to Cary after Brad agreed to get her a car and to start trying to have children.
Brad Cooper is on trial for allegedly killing Nancy. Her body was discovered next to a storm water retention pond in an unfinished subdivision July 14, 2008 - two days after she was reported missing. An autopsy showed she was strangled. Prosecutors say Brad killed Nancy because he was angry she planned to divorce him and move with their two daughters to Canada.
Cooper told detectives Nancy went out for a run the morning of July 12 and never returned. His lawyers have maintained that detectives focused on Cooper from the beginning of their investigation and never looked at other suspects.
On the witness stand Tuesday, Lister said Nancy met the other man while on vacation in Florida, but she had no knowledge of any other extra marital affairs.
The defense maintains that Nancy was involved with a different man in the months before her death and has even questioned the paternity of her second daughter.
Defense attorney Howard Kurtz's final question to Lister was if she had ever been asked to provide DNA samples in relation to the Cooper children.
The prosecution objected and the question was not answered.
The defense has also admitted that Brad had an extra-marital affair. On the stand later Tuesday, Nancy's friend Theresa Hackeling testified about how Nancy learned of that affair.
She said that the woman involved - Heather Metour - was a friend of Nancy's who told her what happened. Hackeling said Brad denied the affair, but later admitted it because Metour was a defendant in an alienation of affection lawsuit and he feared he would be called to testify.
Hackeling was also asked about the sports bra that Nancy was wearing when her body was found. She told jurors that it appeared that the strap was folded under as if it had been pulled on.
The defense objected to the line of questioning, saying it was obvious prosecutors were trying to infer that someone put the sports bra on the body after death.
The judge overruled the objection.
Also Tuesday, Nancy's friend Jenipher Free recounted an argument she witnessed between Nancy and Brad at their daughter's daycare in May 2008. She said they were out in the parking lot yelling at each other and Brad blocked Nancy with his body to keep her from getting to her daughters in his car.
Free said she heard Brad say "You don't own the kids I own them too."
Free said she heard Nancy say "You're just trying to be dad of the year. It's all for show."
Free said after the argument ended that Brad sped off in his vehicle with the tires squealing. She also testified that while she knew Nancy and her children, the incident was the first time she'd ever seen Brad dropping of his daughters at the school.
Another Cooper acquaintance, Shirley Hull, recounted another incident at Triangle Academy in May 2008. She said one afternoon as she was leaving the school, she witnesses Brad and Nancy having an argument in Nancy's car.
"You keep the house. I'm keeping the girls," Hull said she heard Nancy say.
Hull said she could see both of the Cooper children crying.
Testimony Tuesday afternoon continued with a computer security expert from Cisco Systems where Brad Cooper worked until his arrest. He testified that Brad asked him in May 2008 about software to erase all data from a computer.
The Cooper trial is now in its fourth week of testimony. Prosecutors have presented little physical evidence linking Brad to his wife's murder. Instead, much of the testimony has been about the Cooper's relationship and circumstantial evidence.